The present invention relates to a process and an apparatus for balancing rotating bodies, in particular rotors of electric motors.
More specifically the invention relates to balancing of rotating bodies by material removal from the peripheral surface thereof, depending on data supplied by a measurement unit establishing the amount and angular position of the unbalances to be eliminated in balancing planes disposed at right angles to the rotation axis of the rotating bodies.
The detected data is sent to a work unit of the apparatus generally comprising one or more rotating tools, in particular milling cutters, that are operated to carry out material removal in an appropriate amount and position, by one or more milling operations at each balancing plane.
For carrying out rotor balancing in electric motors it is necessary to consider the fact that they are made up of plate-like packs having radial segments spaced apart the same distance from each other, that separate the housing slots of the electric windings and exhibit expansions, at the outer end thereof, extending in a circumferential direction and separated from each other.
Material removal for balancing must be carried out on these expansions that on the other hand have a modest thickness so that it is not possible to penetrate too deeply thereinto with the tool, otherwise the expansions themselves will be weakened and windings will be damaged. This restriction is particularly felt in the case of small motors, such as motors for household appliances, electric tools and the like. Since the unbalance plane is not always coincident with the axial plane of symmetry of the segments, it is not always possible to operate with the tools in the middle of the expansion width, where the greatest material mass is present. It is then necessary to vectorially resolve the unbalance into two or more components in directions constrained by the pitch existing between the expansions, so as to distribute the material removal in the central regions of two or more adjacent expansions.
For the above purpose balancing apparatus are known which are provided with two or more tools, in particular milling cutters, disposed spaced apart from each other on the same rotating shaft extending perpendicular to the rotation axis of the rotor to be balanced , the distance between the tools depending on the pitch of the rotor expansions.
The tool carrier shaft is movable in directions orthogonal to each other and specifically substantially tangential to the rotor and radial thereto, to position the tools on the expansion surface and then make them penetrate thereinto. Furthermore, the shaft is movable parallel to the rotor axis to move the tools forward for material removal.
The rotor is such supported that it can be angularly rotated and fastened in front of the tools at an angular position corresponding to the unbalance or to the vectorial resultant of the unbalance.
Combination of the positioning movements of the tool carrier shaft and the rotor, carried out automatically on apparatus of the known art, based on the unbalance data detected by the measurement unit, allows the tools and the expansions to be brought to such a mutual position that preestablished material amounts can be removed from several expansions, always operating in the middle of the expansion width and in such a manner that the overall amount of the removed material corresponds to the resulting mass to be removed in the unbalance plane.
These apparatus carrying out the so-called vectorial milling, generally allow quick balancing operations, by virtue of the presence of several milling cutters operating simultaneously. On the other hand, depending of the shape of the rotor slots, replacement of the milling tools may be required and this involves slowing down in working.
In addition, although working can be carried out simultaneously with several tools, the restriction imposed by the fact that it is necessary to operate in the middle of the expansions does not always allow the material removal capacity to be maximised.
Furthermore, with these apparatus it is difficult to remedy possible working errors leading to a balancing which is not quite satisfactory or does not fall within the tolerance limits. In fact, a second milling operation asks for the second cut to be exactly superposed on the first one, otherwise the milling cutter does not remove the programmed material alone but it does remove other material too on a side of the first notch, and this precise positioning is of very difficult achievement. Consequently, when the workpieces have been already milled during a first cut and, on checking, they do not fall within the tolerance limits, recovery of same by a further milling operation can be hardly obtained.
Also known are balancing apparatus utilising a milling cutter rotating around an axis transverse to the axis of the rotor to be balanced, which cutter has an outline in the form of a concave arc of a circle of a diameter substantially as long as the diameter of the outer outline of the rotor so that the rotor geometrically fits the milling cutter concavity.
The cutting outline of the milling cutter generally encompasses several rotor expansions, so that it is possible to remove material from several adjacent expansions simultaneously and symmetrically with respect to the unbalance plane.
This system carrying out the so-called polar milling however, has the drawback that it cannot work very deeply, since the tool operates over the whole width of the expansions. In addition, a different tool for each rotor diameter is required. Finally, balancing apparatus are known that are such structured that they can carry out both vectorial milling and polar milling with one and the same tool. These apparatus however need a particular rotating tool, capable of removing material in two different directions on the expansion surface, and related actuating means capable of bringing the tool into engagement with the expansions forming a predetermined angle and of moving it forward during the vectorial milling. Polar milling is on the contrary carried out by rotating the rotor. Since the rotor is rotated under the effort exerted by the tool, it is necessary to provide for an appropriate rotor engagement system by acting on the shaft of the rotor itself.
Under this situation the present invention aims at providing a process and an apparatus for balancing of rotating bodies, in particular rotors of electric motors, that are able to substantially overcome the above described drawbacks.
In particular, it is an aim of the invention to provide a balancing process and the related balancing apparatus that substantially reduce the requirement of replacing the tool when balancing of rotating bodies of different configuration is concerned, in the case of rotors having different slot conformations or different diameters for example.
It is a further aim of the invention to provide a balancing process and apparatus enabling increase of the material removal capacity using the same tool.
A still further aim of the invention is to provide a process and an apparatus of the above specified type enabling balancing of already milled pieces that do not yet fall within predetermined tolerance limits.
Another aim of the invention is to accomplish a balancing process and apparatus that can mostly utilise structural and operating elements already present in known apparatus, so that important modifications of the apparatus themselves is not required.
These aims are achieved by a process for balancing rotating bodies like rotors of electric motors, of the type comprising removing material from surface portions of said bodies, and in which at least one rotating tool is caused to penetrate into said surface portions at an axial unbalance plane of said bodies and is caused to move forward substantially parallel to a rotation axis of said bodies for removing a preset amount of said material, wherein said at least one rotating tool is also caused to carry out a translation along an arched work trajectory for removing material from said surface portions substantially in a circumferential direction of said bodies.
For performing said process there is provided an apparatus for balancing rotating bodies like rotors of electric motors, by removing material from surface portions of said bodies at an unbalance plane passing through a rotation axis of said bodies, comprising a support for at least one of said bodies and at least one rotating tool displaceable along two directions perpendicular to said rotation axis and along a direction parallel to said rotation axis, and further comprising means for causing said at least one rotating tool to carry out a translation along an arched work trajectory lying in a plane substantially transverse to said rotation axis of said bodies.